1. Field of the Invention
This invention finds application in the testing of individual hydrophones mounted inside seismic streamer cables.
2. Discussion of the Prior Art
Marine seismic streamer-cable sections, used in oceanographic studies and marine geophysical exploration consist of a cylindrical plastic jacket wherein are contained a plurality of hydrophones. Such sections are commonly 100-200 feet long. The cable sections are filled with a light-weight insulating fluid such as odorless kerosene to render the sections neutrally bouyant and are then sealed against oil leakage. The sections further include stress members and electrical conductors for transmission of hydrophone signals. A complete seismic streamer cable is made up of a large number of individual sections that are mechanically and electrically connected together.
In use, a streamer cable is towed behind a ship through the water along an assigned line of survey. Seismic signals propagate through the water as pressure waves. Impinging on the streamer-cable jacket, the pressure waves are transferred to the hydrophones, the light-weight fluid serving as a transmission medium.
There may be 25 to 40 hydrophones within each sealed cable section. The hydrophones are usually electrically connected in parallel to form one or a number of groups of hydrophones, each group constituting a separate data channel. Series-parallel connections are also known. Signals from the individual data channels are transmitted to a signal utilization device by the previously-mentioned electrical conductors.
The hydrophone spacing depends on the length of the cable section as well as the number of hydrophones contained therein. Spacings may range from one to several feet. The hydrophones within any one section may be connected in parallel to a dedicated pair of conductors to form a single group or data channel. In other configurations, the hydrophone may be connected as two or more separate groups, or hydrophones from adjacent cable sections may be included as single group. See for example, U.S. Pat. Nos. 3,133,262, 3,436,722, 3,441,902 and 4,092,629, all assigned to the assignee of this invention and which are incorporated herein by reference as a showing of various known cable configurations.
In order to make meaningful, quantitative comparisons of the signals from each group, it is essential that the electrical responses of the respective groups, as well as the responses of the individual hydrophones, be uniform throughout the length of the streamer cable. It is evident that a dead or shorted-out hydrophone in a particular group will adversely affect the electrical response of that group.
As discussed above, the hydrophones are sealed inside an oil-fitted jacket. Accordingly, it is not practicable in the field to remove the hydrophones from the jacket to physically check each individual hydrophone quantitatively. It is desirable to test each hydrophone in situ. It has long been known to manually tap the outside of streamer cable jacket in the vicinity of a hydrophone by means of the handle of a screwdriver or like tool. The applied blow generates a positive pressure pulse that excites the hydrophone to be tested and, unfortunately, usually to a lesser extent, one or more adjacent hydrophones. The resulting hydrophone output pulse qualitatively indicates that the hydrophone is alive or dead and indicates the signal polarity thereof. Since a manually-applied tap is never uniform from hydrophone to hydrophone, that method is too crude to quantitatively compare important hydrophone response characteristics such as relative sensitivity and natural frequency.
In another method of in-situ testing, an electrical square wave of indefinite period is applied to each group as a whole. The resulting output transient represents the response of an entire group but nothing is learned about the responses of the separate hydrophones that make up the group.
In yet another test procedure the entire cable section is placed in a sealed test tank. The section is then subjected to fluctuating pressures of a pulsating air column, the pulsation frequency being varied over a desired range. But again, the resulting output signal represents the combined response of the entire group.